Gas turbine engines are a primary source of power for aircraft propulsion. The main components of a gas turbine engine include a compressor section, a combustor section and a turbine section. Stationary vanes, disposed between rings of moving blades within the turbine section, direct and stabilize gas flow from one stage of moving blades to the next. The stabilization of gas flow optimizes the amount of work extracted from hot gases in the turbine section.
The severe operating conditions under which portions of a gas turbine engine operates causes component surfaces to deteriorate. Exposure to the hot gases causes deterioration due to cracking, corrosion, oxidation, and erosion. The high temperature and pressure under which a gas turbine engine operates also causes creep distortion of turbine blades and vanes. As a result, turbine efficiency is lost and the vanes and blades must be repaired or replaced. Due to the high cost of these engine components, it is desirable to repair the components for subsequent use rather than replacing them. Repairing the components, however, may result in additional component surface degradation due to the effects of removing surface contamination and/or protective coatings in the repair process. Conventionally, the vanes and blades may be treated with HF gas to remove oxides from damaged areas. Other acid treatments and mechanical abrasion techniques may also be used for cleaning. The above cleaning techniques, however, may erode component surfaces and result in additional component defects.
In the prior art, many attempts have been made to repair defects in these engine components. For example, fusion welding has been used to repair cracks and other defects, however, additional cracking has often occurred related to rapid heating and cooling. Brazing techniques have also been employed to repair defective areas. Difficulties encountered with this technique and variations thereof include the inability to completely remove contamination in the cracks and inability to completely fill narrow cracks with braze material.
Another repair process is described in U.S. Pat. No. 4,008,844, which is assigned to the present assignee and incorporated herein by reference. According to this patent, a mixture of metal powders is made from two powders with different compositions. One composition approximates that of the superalloy to be repaired while the other composition also approximates that of the superalloy to be repaired, but contains a melting point depressant, usually boron. The mix has a paste like consistency. The defect to be repaired is filled with a mixture of these powders and then heated to a temperature at which the boron containing powder melts, but the boron-free powder and the substrate do not. Solidification then occurs isothermally over a period of time as the boron diffuses into the substrate thereby raising the solidification temperature of the melted constituent. This process is successful, but is limited in that it is difficult to apply exactly the right amount of material to shallow surface defects. In addition, when larger defects are to be covered or filled, the excess molten tends to flow away from the defect during the heat treatment process.
Accordingly, there exists a need for a more controlled surface repair coating applicable to superalloy articles, such as gas turbine engine components.
The objects of the present invention are to (1) provide a surface build-up repair coating applicable to superalloy articles, such as gas turbine engine components, and (2) provide a repair process particularly suited for repairing cracks and shallow crevices in superalloy articles, such as engine components, so that minimal or no subsequent grinding or buffing steps are required to return the article to its proper dimensions.